In general, a mobile communication system has been developed for the purpose of providing communication while securing mobility of a user. Such a mobile communication system has reached a high-speed data communication service as well as a voice communication thanks to the rapid development of technique. Under this background, various techniques to improve communication speed using multiple antennas have been discussed.
Much research into the technology using multiple antennas has been conducted, and the technology is used in real life by being applied to various wireless communication systems such as the cellular communication network, the wireless local area network (WLAN) and the like. The technology using multiple antennas is largely classified into two categories. The first is to increase the transmission rate by transmitting multiple streams simultaneously using multiple antennas, and the second is to increase transmission success probability by using spatial diversity.
Gain through the spatial diversity can be obtained by using more antennas in number than streams to be transmitted. Due to the multipath fading effect of the wireless communication, the antennas experience different channels, thereby securing diversity in the channels. In this way, a gain of diversity is obtained by selectively using a good antenna for the channel. If such a diversity gain is obtained, the average channel state improves and the transmission success probability increases, which results in increase in communication efficiency.
When antennas are selectively used, the most ideal case is when channel information for each antenna is known in advance. However, when selecting a transmission antenna, only the receiving side is exactly aware of channel information. In order to overcome this problem, a number of methods of properly selecting an antenna have been devised.
One simple method is for the receiving side to measure channel information and provide feedback to the transmitting side. A method for feeding back channel information is already defined in IEEE 802.11n for MIMO communication. However, there is a limit because overhead required for channel information feedback is very large and wireless channels changes rapidly. In addition, if an antenna combination that is not used for transmission exists, channel information cannot be known.
Another method is for the transmitting side to directly measure channel information. That is, under the assumption that both channels (or uplink channel and downlink channel) are the same, channel information coming from the receiving end is used for transmission using channel reciprocity. However, since characteristics at the radio frequency (RF) chain stage change depending on the terminal, correction (calibration) is required to maintain the channel reciprocity, which also leads to large overhead.
In order to overcome the limitation of using channel information such as large overhead, there is a method of selecting an antenna in a trial-and-error manner. According to the trial-and-error selection method, when transmission failure occurs (no MAC ACK is received) in an antenna, which has been continuously using for, another antenna is selected and used. In conclusion, the antenna is selected and used depending on whether the most recent transmission or reception is a success/failure.